The external combustion engine of the above type is recently called as a steam engine of a liquid piston type, which is known in the art, for example, as disclosed in Japanese Patent Publication No. 2007-255259. According to such a known steam engine, working fluid of liquid condition is charged into a pipe-shaped main container and the working fluid is movable in the main container. A portion of the working fluid is heated by a heating portion, which is provided at one end of the main container, to vaporize the working fluid. Vaporized working fluid (steam of the working fluid) is then cooled down by a cooling portion, which is provided at an intermediate portion of the main container, to condense the steam to the liquefied working fluid. The liquid portion of the working fluid is periodically displaced (so-called, self-excited vibration) by alternately repeating the vaporization and condensation of the working fluid, so that kinetic energy is taken out from the self-vibration for the liquid portion of the working fluid at an output portion communicated to the other end of the main container.
According to the above steam engine, working fluid is also charged into an auxiliary container, which is a separate container from the main container, and the main container and the auxiliary container are communicated with each other through a restricted portion. According to such a structure, inside pressure of the main container is adjusted by use of the auxiliary container, in order to improve output and efficiency of the external combustion engine.
FIG. 3 is a schematic view showing an outline structure of an external combustion engine (a steam engine). The steam engine of FIG. 3 is shown in this application as a reference example, for the purpose of explaining not the prior art but the present invention. In other words, the steam engine does not belong to a prior art.
In FIG. 3, multiple (three) main containers 12 to 14 are connected to one output portion 21. Namely, an external combustion engine is shown as a liquid-piston type steam engine having multiple cylinders.
According to the reference example, a phase of the movement of the working fluid 11 differs from each other among the multiple main containers 12 to 14, so that mechanical vibration at the output portion 21 is reduced.
According to the reference example, the working fluid 11 is charged in a casing 29 of the output portion 21, a casing 29 is communicated with the main containers 12 to 14 through a first communication pipe 33, and restricted portions 35 are formed in the first communication pipe 33. According to such a structure, the casing 29 demonstrates a function of the auxiliary container, as disclosed in the above publication (No. 2007-255259).
Since the working fluid 11 is also charged in the casing 29, air in the casing 29 is prevented from flowing into the main containers 12 to 14 through minute gaps between pistons 22 to 24 and cylinders 25 to 27 of the output portion 21.
Furthermore, according to the reference example, the casing 29 is communicated with the main containers 12 to 14 through a second communication pipe 34, which is arranged in parallel to the first communication pipe 33, and the second communication pipe 34 is opened or closed by a valve 38.
FIG. 4 is a time chart showing an operation of the engine at its starting period according to the above reference example. The starting period is divided into two steps, one is a motoring step and the other is a start-up step. In the motoring step, the pistons 22 to 24 are driven by an outside driving power for one cycle. In the start-up step, the output (the rotational speed) is increased to a predetermined output value (a predetermined rotational speed), after the motoring step has ended. When the start-up step is finished, the steady state operation starts, during which the predetermined output (electrical power) can be taken out from the engine 10.
A certain amount of the working fluid 11 in the main containers 12 to 14 is drained off to the casing 29 through the second communication pipe 34 by opening the valve 38 during the motoring step. So-called liquid-drain-off is carried out.
When the above liquid-drain-off is carried out, the working fluid 11 returns from the main containers 12 to 14 to the casing 29. However, an excessive amount (an amount more than necessary) of the working fluid 11 may return to the casing 29 during the liquid-drain-off process. Then, the working fluid 11 gradually flows back into the main containers 12 to 14 through the restricted portions 35 after the liquid-drain-off. As a result, the liquid amount of the working fluid 11 in the main containers 12 to 14 becomes to an adequate amount. When the liquid amount of the working fluid 11 in all of the main containers 12 to 14 has become to the adequate amount, the starting operation is finished and changed to a steady state operation.
However, as seen from FIG. 4, according to the above reference example, a time necessary for the start-up step (the start-up time) becomes longer, because the phase of the movements in the main containers 12 to 14 differs from each other. As a result, it is a problem in that heat loss during the start-up step may become larger.
We could make a flow passage area of the restricted portions 35 larger, as one of counter measures for the above problem. However, according to such countermeasure, in the main container, in which the operational phase is in the most advanced condition, the working fluid 11 may excessively flow into the main container. After all, the start-up time may become longer even in such a countermeasure.